You can see in the video, it's not a "belay device". It's a "self-belay device". The climber has control of his own travel on the rope.

The video doesn't show the top anchor, but I guess the rope goes through a simple pulley. That also means that the weight load on the top anchor is body-weight only. Half of a regular belayed anchor, and no dynamic falls. Although I don't know how those guys controlled their pendulum so gracefully.

I think it would be great fun! I'd love to set that rig up on any cliff anywhere, and explore the rock-face like I've never been able to do before!

The Unicender is a much better tool since it can be used on single rope technique as well as doubled rope technique. For arbos DdRT is NOT two ropes. One rope is used with it being 'doubled' over a limb or tie in point.